The catalytic partial oxidation of hydrocarbons, natural gas or methane to synthesis gas has been processed for many years. While currently limited as an industrial process, the partial oxidation is of interest for the significant released heat and for the use of smaller reactors.
European patent application EP 0 725 038, discloses materials having a layered structure of the Hydrocalcite type, in which rhodium is inside the interior of said structure, which can be represented by the general formula:[RhaRubXcYd(OH)2]Z+(An−z/n)mH2Owherein X and Y are divalent or trivalent metal cations,    0≦a≦0.5; 0≦b≦0.5; 0.5≦c≦0.9; 0≦d≦0.5 and a+b+c+d=1,    m is 0 or a positive integer,    A is a hydroxyl or any anion or anionic complex having n electrical charge.    z is the total electrical charge of the cationic component.
International application WO 01/25142, discloses a catalyst which is obtained from an Hydrotalcite-type precursor containing Ni, which is worked in a reforming process using steam and/or CO2.
International application WO 01/53196, discloses a catalyst which consists in a refractory fibrous structure comprising a plurality of ceramic oxide fibres and at least one active catalyst element, chosen among Rh, Ni and Cr, supported on said fibrous structure. Such a catalyst is claimed to better resist a thermal shock, than the conventional supported catalysts do.
International application WO 01/28679 discloses a catalyst which consists in a mixture of at least two different metal carbides (especially Mo, W, Cr), which optionally include an additional promoter and/or a catalyst support. It is claimed that no appreciable coking occurs, that the catalyst deactivation is avoided or at least delayed, and that this catalyst can be industrially worked under better economical conditions than the conventional catalysts do.
International application WO 03/000398, discloses a catalyst which consists in a classical catalytic active phase such as a metal transition element (Ni, Mo, Rh, Pt, . . . ), which is supported on a silicon carbide having a high specific surface area less or equal to 100 m2/g. The contact time between the gaseous hydrocarbon, the oxidizing gas, optionally in the presence of a small amount of water, and the silicon carbide catalytic support, is greater than 0.05 s, the temperature greater than 800° C. and the pressure inside the reactor greater than the atmospheric pressure. The advantages of this invention are the use of a new silicon carbide support having a high surface area, typically between 10 and 50 m2/g, with classical active phases.
U.S. Pat. No. 6,458,334 B1 discloses a catalytic partial oxidation process involving the use of a classical metal catalyst (Ni, Co, Ir, Pt, . . . ) or a combination of them thereof which is supported on or in a ceria monolith. The pressure is between 105 Pa and 20.105 Pa (1 to 20 bar), the Gas Hourly Space Velocity (GHSV) is of about 50,000 to 500,000 hr−1.
However, none of the known existing catalytic partial oxidation processes are able to reach a sufficiently high conversion rate of reactant gas. Moreover, a high selectivity of CO and H2 reaction products can only be reached with the use of a large amount of rare and costly catalysts, or with taking the risk of an excessive coking or of a premature failure due to a lack of heat resistance and a mechanical instability of cheaper catalysts on the support structure.
There is indeed a continuing need for new catalysts that are mechanically stable, with high surface area, preferably from 10 to 300 m2/g, and that retain a high level of activity and selectivity to CO and H2 products under conditions of high temperature, without excessive coking.
In the International application PCT/IB03/01673 (WO 03/099436) filed on Apr. 30, 2003, the inventors claimed a composition to overcome the above-mentioned drawbacks, which may thus be used as a catalyst for partial oxidation of hydrocarbons and which essentially consists in a solid solution of a mixture of at least a magnesium oxide type phase compound and at least a magnesium silicate type phase compound in which Al, and Rh and/or Ni cations are soluted.
The precursor of this composition is a hydrotalcite-type structure. After calcination at 900° C., two main phases are present: magnesium oxide type phase, a magnesium silicate type phase (forsterite-type), in which Al and the cation of the active phase (Rh and/or Ni) are soluted.
The composition claimed in PCT/IB03/01673 can be prepared from a precursor containing active metals of VIII group (Ni and/or Rh) and silicates as anions having a structure that is referred to as “hydrotalcite-like” (HT). Hydrotalcite-like compounds are anionic clays, that have a sheet-like structure. The sheets are separated by anions which balance the net positive charge of the sheets. In the context of the present invention, the anions of the anionic sheets are silicates or polysilicates and in the cationic sheets are present Ni or Rh, or a combination of those. The materials obtained by calcination of said Hydrotalcite-like compounds have high thermal resistance and are very stable. After an activation procedure, they are very active and do not show any carbon formation in the catalytic partial oxidation process.
More specifically, this composition is prepared from an HT precursor represented by the general formula (I):[RhxNiyMgiAlm(OH)2]z+(An−z/n)kH2O,  (I)wherein An− is mainly a silicate or a polysilicate anion;
0≦x≦0.3;
0≦y≦0.9;
0≦l≦0.9;
0≦m≦0.5;
0≦k≦10;
x+y>0;
0.5≦y+l≦0.9;
x+y+l+m=1; and
z is the total electrical charge of the cationic element.
In a preferred embodiment of this composition,
0≦x≦0.1;
0≦y≦0.3;
0.3≦l≦0.8;
0.1≦m≦≦0.4;
0≦k≦5;
x+y>0;
0.6≦y+l≦0.8;
x+y+l+m=1.
Among these above mentioned HT precursor, the following compounds are the most preferred:
[Ni0.08Mg0.60Al0.32(OH)2]0.32+(SiO32−)0.16kH20,
[Ni0.08Rh0.0015Mg0.60Al0.3185(OH)2]0.32+(SiO32−)0.16kH20,
[Rh0.005Mg0.71Al0.285(OH)2]0.32+(SiO32−)0.16kH20,
[Ni0.01Rh0.0002Mg0.67Al0.3198(OH)2]0.32+(SiO32−)0.16kH20,
[Ni0.02Mg0.63Al0.35(OH)2]0.35+(SiO32−)0.175kH20,
[Rh0.0004Mg0.65Al0.3496(OH)2]0.35+(SiO32−)0.175kH20,
[Ni0.02Mg0.78Al0.20(OH)2]0.35+(SiO32−)0.175kH20, and
[Rh0.0004Mg0.80Al0.1996(OH)2]0.20+(SiO32−)0.10kH20.
In order to improve the stability of this composition and to improve its selectivity, the inventors have tried to develop a process to support the above-mentioned composition (the active phase) on an inert support.
They however found that working a classical deposition process on standard catalytic supports, such as alumina, zirconia, silicon carbide or magnesium oxide, was not efficient. In fact the deposition of the hydrotalcite precursor on α-alumina beads did not tie with the support, the active phase being separated from the beads, the same occurred with commercial silicon carbide which have an average specific area of less than 5 m2/g, and the tentative with ZrO2 pellets resulted in the crash of the pellets during the preparation.
That is why they develop a new process on the “form memory” concept, to increase the interaction between the support and the active phase in order to improve the stability of the resulting to high temperature. This concept involves the use one raw material (O), which owns one or several chemical elements (for example A), which is still present in the final product after synthesis. This raw material, which can have several geometric forms (pellets, beads, honeycomb, filter, tube, . . . ) and several architecture/microstructures (high surface area, porosity, pore size, . . . ) is attacked by chemical reactions (solid-liquid and/or solid-gas and/or solid-solid reactions) with precursors (B, C for example), which must also be present in the final product. The final product is a new material (ABC for example), which is supported on the initial raw material (O).
In the best case, one or several layers of the new material (ABC) are developed around a core of the raw material.
Such a concept was first disclosed in U.S. Pat. No. 4,914,070 and related to the production of silicon carbide with high surface area for catalyst applications. This patent disclosed a process for the production of fine grains of silicon carbon, which are formed by reacting SiO vapour on carbon. SiO vapour is obtained by heating a mixture of SiO2 and Si at a temperature between 1100 to 1400° C. This vapour attacks reactive carbon with a high specific surface area (more than 200 m2/g) in a second zone at temperature between 1100 to 1400° C. The final product issued from the reaction between SiOgas and Csolide is silicon carbide with high surface area (more than 100 m2/g) with or without a carbon core. The main advantages of this process are the production of silicon carbide with high surface area while keeping the initial geometry and the architecture/microstructure of the raw carbon.
The ideas of “dissolution/precipitation” method similar to that described new section are developed to build hydrotalcites of two Congresses ICC (6th and 11th International Congresses on Catalysis) in Baltimore (1976, 1996).
Papers of van Dillen J. A., Geus J. W., Hermans L. A. and van der Meijden J. (1976, 6th ICC) described a method of production of supported copper or nickel catalysts by “deposition-precipitation”. The support, which reacted with the nickel and the cobalt precursor in solution to form an hydrotalcite, was SiO2. Penetration of nickel ions into the silica support or migration of the silica, lead to thicker nickel hydrotalcite layers. Conversion of an appreciable fraction of the support into a compound having a layer structure profoundly affected the texture of the support.
Papers of Espinose J. B. and Clause O. (1996, 11th ICC, p 1321-1329) described the promotion of -alumina dissolution by metal ions during impregnation and the thermal stability of the formed coprecipitates. The metallic elements were nickel and cobalt. The method developed was the “deposition-precipitation” and the support was -alumina. As described in the 6th ICC the Al can reacted in solution with the Ni(II) ions or Co(II) to form an hydrotalcite structure. The experiments allow the separation of the secondary phase—hydrotalcite—from the mother oxide support, alumina.
However, the authors suggested that, the supported hydrotalcites were, in fact, weakly bound to the surface and free to move away from alumina once formed. In both papers, no chemical reaction was studied using this new type of “active support”.
That is why the inventors developed a new combination which overcomes the above mentioned drawbacks.